This invention relates to a diffusion bonding method for corrosion-resistant materials. More particularly, it relates to a diffusion bonding method for Ni base or Fe base high alloy corrosion-resistant materials.
There are many Ni base or Fe base high alloy corrosion resistant materials. These materials exhibit excellent corrosion resistance due to a high content of Ni and/or Cr and are able to withstand relatively severe corrosive environments. When these materials are used as construction materials, they are frequently joined by welding. However, when welding high-alloy materials, weld cracks are inevitable.
Among the most typical high alloy corrosion resistant materials which suffer from weld cracks are Ni base and Fe base corrosion resistant materials, such as those used for high temperature equipment, and Ni base and Fe base clad steel tubes using stainless steel or a high alloy as a cladding material.
In thermal power stations, nuclear power stations, and chemical plants, many types of equipment must be operated at high temperatures. The materials used for such high temperature equipment must have many different properties, including resistance to oxidation by steam, high temperature corrosion resistance, high temperature strength, creep rupture strength, and good fatigue strength. For example, a cracking tube for use in equipment for the manufacture of ethylene is used at a temperature of 800 degrees C. or higher, so it must have high temperature corrosion resistance and excellent creep rupture strength.
High alloy materials for such high temperature equipment have conventionally been joined to one another by welding. However, as these materials have a high carbon content or they contain a large number of alloying elements, they are highly susceptible to weld cracking, and it is difficult to avoid a decrease in the high temperature strength of the welded joint. TIG welding, in which the amount of heat applied during welding is suppressed, has been much used for welding such materials, but weld cracking can still not be completely prevented, and the welding efficiency of TIG welding should be low to avoid the occurrence of weld crackings.
Chemical plants and pipelines frequently use so-called stainless steel clad tubes with an inner tube (cladding) of stainless steel for transporting corrosive liquids. These clad tubes are formed by combining an outer tube of low alloy steel or carbon steel with an inner tube of stainless steel to form a clad assembly. The inner tube of stainless steel provides corrosion resistance against the liquid flowing through the assembly, while the outer tube of low alloy steel or carbon steel gives the clad assembly the required strength. Stainless clad steel tubes of this type have been found to be economical and practical. Stainless clad steel tubes having an inner tube of low alloy steel or carbon steel and an outer tube of stainless steel are also frequently employed in chemical plants and the like when only the outside of the clad assembly requires corrosion resistance.
However, when stainless clad steel tubes are joined by conventional welding methods, various problems occur, so it is necessary to exercise special care when welding these materials. For example, Japanese Published Unexamined Patent Application No. 58- 167094 discloses a welding method for a multi-layer tube in which a different welding material is used for each layer.
However, even when such a welding method is employed, it is very difficult to perform circumferential welding of clad steel tube. Some of the problems of performing a circumferential weld on a stainless clad steel tube having a stainless steel tube on the inner surface of the clad assembly will be described below.
Circumferential welding of a steel tube is normally carried out from the outside of the tube, and metal having a similar composition as the tube is used as the welding material. However, when steel tube with internal stainless cladding is welded from the outside by single-side welding, if a stainless steel welding material is used to weld the internal cladding and carbon steel welding material is used to weld the external base metal, when the base metal is welded, a portion of the stainless steel welding metal for the internal cladding, which has already been welded, is remelted. The remelted stainless steel welding metal is diluted by the carbon steel welding metal and solidifies to form a hardened layer. As a result, cracks form in the weld portion, and depending on the composition of the stainless steel, weld cold cracks and hot cracks are formed. On the other hand, when a high-alloy welding material is used to weld both the inner and outer tubes, there is the possibility of insufficient weld strength. Furthermore, stainless steel appears on the outer surface of the outer tube, causing galvanic corrosion due t the contact of the stainless steel and the carbon steel.
Even if a welding method like that disclosed in Japanese Published Unexamined Patent Application No. 58-167094 is employed, the welding material and welding conditions must be successively changed for the inner layer, the intermediate layer, and the outer layer, so welding becomes complicated. Furthermore, in that method, the soundness of the weld depends on the skill of the welder, so the weld quality is highly variable. A bigger problem is that in order to prevent weld defects, it is necessary to restrict the welding speed, so the efficiency of welding is extremely poor.
Another method which can be used to join high alloy corrosion resistant materials is diffusion bonding. Diffusion bonding has the advantages that it produces almost no deformation during bonding and that heat effects are small. However, conventional diffusion bonding methods have the drawbacks that they are very time consuming and inefficient. Moreover, since diffusion bonding must often be carried out under a vacuum, equipment costs tend to be high.